[GitHub] [arrow-rs] jorgecarleitao commented on a change in pull request #1248: Specialized filter kernels

[GitBox <gi...@apache.org>]

jorgecarleitao commented on a change in pull request #1248:
URL: https://github.com/apache/arrow-rs/pull/1248#discussion_r800191885



##########
File path: arrow/src/compute/kernels/filter.rs
##########
@@ -119,17 +155,83 @@ impl<'a> Iterator for SlicesIterator<'a> {
     }
 }
 
+/// An iterator of `usize` whose index in [`BooleanArray`] is true
+///
+/// This provides the best performance on all but the least selective predicates (which keep most
+/// / all rows), where the benefits of copying large runs instead favours [`SlicesIterator`]
+struct IndexIterator<'a> {
+    current_chunk: u64,
+    chunk_offset: i64,
+    remaining: usize,
+    iter: UnalignedBitChunkIterator<'a>,
+}
+
+impl<'a> IndexIterator<'a> {
+    fn new(filter: &'a BooleanArray, len: usize) -> Self {
+        assert_eq!(filter.null_count(), 0);
+        let data = filter.data();
+        let chunks =
+            UnalignedBitChunk::new(&data.buffers()[0], data.offset(), data.len());
+        let mut iter = chunks.iter();
+
+        let current_chunk = iter.next().unwrap_or(0);
+        let chunk_offset = -(chunks.lead_padding() as i64);
+
+        Self {
+            current_chunk,
+            chunk_offset,
+            remaining: len,
+            iter,
+        }
+    }
+}
+
+impl<'a> Iterator for IndexIterator<'a> {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while self.remaining != 0 {
+            if self.current_chunk != 0 {
+                let bit_pos = self.current_chunk.trailing_zeros();
+                self.current_chunk ^= 1 << bit_pos;
+                self.remaining -= 1;
+                return Some((self.chunk_offset + bit_pos as i64) as usize);
+            }
+
+            // Must panic if exhausted early as trusted length iterator
+            self.current_chunk = self.iter.next().expect("IndexIterator exhausted early");
+            self.chunk_offset += 64;
+        }
+        None
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+/// Counts the number of set bits in `filter`
 fn filter_count(filter: &BooleanArray) -> usize {

Review comment:
       not sure why this was here, but isn't this equal to `filter.len() - filter.null_count()`?




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